To make bronze, tin flakes do a wild dance

Overlaid microscope images show zigzag path (light blue, starts at arrow) of tin island (dark blue), dropping bronze spots (red) on copper (yellow). Schmid, et al./Science

Jitterbugging flecks of metal are challenging some prevailing ideas of how alloys form.

When deposited atop a pure copper crystal, tin atoms form into 100,000-atom rafts that scoot around madly, depositing bronze spots in their wake, physicists at Sandia National Laboratories in Livermore, Calif., have found.

So much for the long-held notion of placid patches of tin slowly trading atoms with the underlying copper to make bronze. Instead, the agitated tin scraps grab copper atoms as they scurry along, report Andreas K. Schmid and his colleagues in the Nov. 24 Science.

“This is so wild and unlikely, you’d never guess it would happen,” exclaims Rudolf M. Tromp of IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. Yet in all likelihood, there are other materials besides bronze that form this way, he adds.

Individual atoms often scoot across a surface, but even pairs moving together are unusual, comments Miquel B. Salmeron of Lawrence Berkeley (Calif.) National Laboratory. “Finding that they move in groups of thousands as a block—that’s remarkable, quite remarkable,” he marvels.

Schmid, Norman C. Bartelt, and Robert Q. Hwang spotted the action thanks to a specialized microscope. A relatively new and extremely rare type of instrument, their low-energy electron microscope records highly magnified images at video speeds. The team also used a scanning tunneling microscope (STM), which can unveil even finer, atomic-level detail but at a slower pace (SN: 10/24/98, p. 268: The original motivation for the work was to better understand the alloying of tin and lead in solder used in nuclear-weapons circuitry.

In previous studies with only the STM, the group had observed that tin atoms assemble into islands if sprinkled across a copper surface. To the scientists’ bewilderment, however, those clusters of thousands of tin atoms would suddenly disappear.

“It’s like looking at a mountain range, turning your back for a second, then looking back and the mountain range isn’t there,” says Bartelt. The STM was too slow to reveal that the tin had simply skittered away. The new microscope cleared up the mystery, he says.

What’s propelling these islands? Sandia’s team proposes a repulsive force between tin atoms in the islands and tin atoms in the immobile bronze spots dotting the surface. The force would push the tin islands away from those spots.

That leads to some surprisingly lifelike behavior, Bartelt says. Each island acts like a “creature [that] . . . searches out clean copper to eat,” he says. Roving tin patches hesitate when their best path is not obvious. Also, he adds, when blocked by lines of repulsive bronze dots, “they do very funny things,” such as shattering or squeezing through as a narrow thread.

Surface alloys serve important technological roles as catalysts or magnetic materials, for instance (SN: 1/28/95, p. 53). So, the new insights into the surface-alloying process—if the observed dancing around turns out to be common—could have widespread scientific and practical impact.

Harnessing the means of propulsion alone might ultimately prove useful, argue Danish physicists Flemming Besenbacher of the University of Aarhus and Jens K. Nørskov of the Technical University of Denmark in Lyngby in an accompanying commentary in Science. They call the propulsion mechanism a “paradigm for a new class of nanomotors,” which may someday power extremely tiny devices, such as switches or pumps.

Click here to watch a movie of a tin raft’s wild dance as it makes the tin-copper alloy bronze. The scurrying black blotch is about 100,000 atoms of tin gathered together in a thin, mobile sheet. This tin island roams on a copper surface, picking up copper atoms and leaving behind bronze spots. At room temperature, the island actually moves more slowly than it seems in the video. That’s because this clip, taken with a low-energy electron microscope, compresses motion that took place over a 4-minute period. However, heating the copper surface by 10°C speeds up the tin raft’s action to about as fast as seen here, the researchers say.

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